1. Technical Field
This invention relates generally to encoding pictorial imagery for reproduction on binary display and/or printing systems, and more particularly to maximizing the number of gray levels for low density segments of an image while reducing screen texturing and increasing apparent resolution in high density segments of the image.
2. Background Art
Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution, measured in terms such as marks per unit length (typically dots per inch). It has been common to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye gives a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density.
Continuous tone images are simulated by organizing groups of sub-elements into j.times.k matrix halftone cells, where j and k are positive integers. The halftone cells have gray level capabilities equal to the number of sub-elements in the cell plus one.
Halftone image processing algorithms are evaluated in part by their capability to deliver a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another measure of image processing algorithm merit is the tendency to produce visual details in the output image that are not part of the original image, but are the result of the image processing algorithm. Such details are called artifacts, and include grain, false contours, and false textures. False textures are artificial changes in the image texture which occur when input gray levels vary slowly and smoothly and the output generates an artificial boundary between the textural patterns for one gray level and the textural patterns for the next gray level. False contours are the result of gray scale quantization steps which are sufficiently large to create a visible density step when the input image is truly a smooth, gradual variation from one to the other.
Briefly, several of the commonly used processing algorithms include fixed level thresholding, adaptive thresholding, orthographic tone scale fonts, and electronic screening. The present invention is concerned with the latter, electronic screening. The factors relating to the screen which determine the type of image to be reproduced include optical density screen values, screen frequency, and screen angle.
There are many formats for electronic halftone cells at various screen angles and screen frequencies. FIG. 1, which is reproduced in part from U.S. Pat. No. 4,4.13,286, shows a screen 10 generated by the repetition of a screen cell 12 in directions "a" and "b". Cell 12 is included between two adjacent parallel lines in direction "a" and two adjacent parallel lines in the direction "b". In FIG. 1, distances .lambda..sub.a and .lambda..sub.b are the spatial frequencies of screen 10 in directions "a" and "b", respectively.
It is generally recognized that the perceived quality of a halftone image is directly proportional to the number of gray levels available, and that a greater number of gray levels are attainable as the screen frequency .lambda. decreases. Further, lower screen frequencies reduce grain and density contouring.
However, reduction of screen frequency is limited by the increased appearance of screen texture and by a lower apparent resolution of the system. Accordingly, prior art systems were designed as a compromise, trading off increased gray scale levels, reduced grain, and reduced countouring for increased texture and decreased resolution.